In the South Park Movie when Kenny says his line at the end, he sounds like Kyle with a higher voice (it was voiced by Mike Judge, but he sounded like he was imitating what he really would have sounded like). But in this episode, Kenny doesn't sound like Kyle at all. So why would Mike Judge impersonate Matt Stone to sound like Kyle, but higher, if Matt himself didn't even bother?

Eric Stough is a producer and animation director of the show. He's been with the show since the beginning. I met him a week ago and we talked briefly about Kenny making an appearance completely unmuffled and undisguised, which hasn't happened (simultaneously) since the movie.